1. Technical Field
The present system and method generally relate to data transmission technology, and more particularly to a system and method for delivering data over multiple twisted pair conductors.
2. Description of the Background Art
Digital Subscriber Line (DSL) technology has existed for several years. There are some significant competing broadband technologies, such as cable and satellite TV, and wireless technologies such as MMDS.and LMDS. Some implementations of these competitive services may be better suited to so-called Video-On-Demand (VOD) services than current DSL technology, but not well suited to traditional data communications or even interactive voice and video applications, where DSL may have the advantage of greater upstream bandwidth and lower latency.
Hence, there is a need to improve DSL technology data rates so as to potentially deliver such services as VOD in an effective manner: DSL service providers, therefore, generally need to increase the data rate by about a factor of two to provide VOD services to a large percentage of their customers. Preferably, this goal may be accomplished without making significant changes to the loop plant.
As those skilled in the art will appreciate, a common implementation of ADSL uses the frequency range from about 30 to 1100 kHz for both downstream (central office to customer premises) and upstream (customer premises to central office) transmissions. Most systems use band separation, with 138-1100 kHz for downstream and 30-120 kHz for upstream. The low frequencies from 0-30 kHz are reserved for Plain Old Telephone Service (POTS), although POTS signals typically reside below about 4 kHz.
The range of frequencies actually used for downstream transmission depends on the length and composition of the particular loop in question. A filter (POTS splitter) is installed at the NID to separate POTS frequencies from ADSL frequencies. The higher frequency content is directed to an ADSL modem via dedicated wires from the POTS splitter.
ADSL modem performance may be limited by several factors: cable attenuation, gaussian noise, Near End Crosstalk (NEXT), Far End Crosstalk (FEXT), impulse noise, dispersion, and other factors. Additional details regarding ADSL technology are know to hose skilled in the art and may be found in ADSL. Standards Implementation, and Architecture by Charles K. Summers, June 1999 ISBN 084939595X, the disclosure of which is hereby incorporated by this reference.
Video on Demand service typically requires at least 2 mbps per channel, and it is desirable to deliver at least two independent channels per customer premises. It is also desirable to deliver Voice over ADSL (VoDSL) and reasonably fast Internet access over the same system. Therefore, a need exists for a DSL service with a data rate of about 5 mbps or more.
The present system and method generally provide data service over a multiple twisted pair conductors. In one embodiment, DSL signals are transmitted from a single transmitter to a single receiver over multiple local loops. Providing DSL service to a single CPE device over multiple loops, data rates significantly higher than those associated with single-loop transmissions are attainable.
Pursuant to another aspect of the present system and method, signals, such as DSL signals, are transmitted over an additional channel defined by the difference in average voltage between first and second twisted pairs. That is, in addition to transmitting data over the first and second twisted pair conductors, data is also transmitted over an additional channel defined by the difference in average voltage between the two pairs, thereby permitting yet additional data transmission capacity.
In one embodiment, a system is provided for transmitting signals over first and second twisted pairs. The system includes a transceiver having first and second line receivers and a line driver. A first transformer is disposed between the first twisted pair and the first line receiver, a second transformer is disposed between the second twisted pair and the second line receiver, and a third transformer is disposed between center taps of the first and second transformers and the line driver. This configuration permits the line driver to transmit signals over a channel existing between the average voltage of the first pair and the average voltage of the second pair.
Conventionally, many buildings, such as houses, have multi-pair cable from a pole to a Network Interface Device (NID) at the house. A large percentage of houses with Internet access already have at least two complete circuits from the central office to the house. According to one embodiment of the present system and method, the two circuits from the central office to the customer premises are generally viewed as a single multi-dimensional medium. Those skilled in the art will appreciate that the system and method described herein logically extend to numbers of circuits greater than two also.
Suppose, for example, that a telephone company customer currently has two copper loops from the central office to customer premises, both being used as POTS lines, one for dial-up Internet access, the other for voice service. This customer could be given two pair DSLxe2x80x94that is a DSL method that simultaneously drives both lines from both directions, with each pair being considered an independent dimension in a multi-dimension modulation scheme. A simple example would be a QAM mapping where the real part of the symbol is mapped to pair 1, the imaginary part of the symbol is mapped to pair 2.
Another example involves Discrete MultiTone Modulation (DMT), used in the above mentioned standard. As those skilled in the art are aware, G.992.1 generates and modulates 256 independent carriers that are spaced by 4312.5 Hz from DC to 1104 kHz. The POTS splitters, separation of downstream and upstream bands, and other considerations reduce the effective number of carriers to about 220 maximum. These carriers are generally capable of carrying up to 15 bits each, or a maximum possible rate of 15*4312.5*220≅14.25 mbps. In practice, 15 bits/carrier may be difficult if not impossible to attain. Instead, 8-10 mbps may be a more realistic limit for short loops, largely due to hardware limitations.
According to another aspect of the present system and method, an ADSL modem is designed to drive two lines simultaneously, thereby potentially doubling speed for the same capacity per carrier. The xe2x80x9ccomplexxe2x80x9d channel can support both positive and negative frequencies, so the medium can support 512 maximum active carriers under the same conditions that will support 256 maximum active carriers in current ADSL. Of course, the same band separation issues still applyxe2x80x94the need for POTS isolation and upstream/downstream frequency separation, so the actual number of active carriers is reduced by some percentage in both cases.
Some advantages associated with some embodiments of the present system and method are as follows. NEXT and FEXT between these two pairs are cancelable because the interferers are from the same encoder. This improves performance relative to two separate modulators independently driving two pairs. Moreover, POTS splitters may no longer be required if VoDSL is assumedxe2x80x94a big obstacle to deployment of current ADSL. This may reduce the need for truck rolls. The telephony services are delivered as a data application on the common ADSL channel. This frees up 30 kHz of additional bandwidth on each pair, equivalent to as much as 600 kbps or so in data capacity.
Additionally, the additional channel, referred to herein as a xe2x80x9cphantomxe2x80x9d, or xe2x80x9cvirtualxe2x80x9d channel, that exists as the difference between the average voltage of pair one and the average voltage of pair two may have more bandwidth than either the 1st or 2nd pair taken individually. There is about half as much resistance per unit length, and less capacitance per unit length also. This virtual channel may be accessed via transformer center taps at each end of the loops.
Importantly, the phantom, or virtual, channel may be a significant benefit. For example the virtual channel could be used for the upstream channel, thereby freeing the downstream channels to use more available bandwidth, which would improve reach significantly. Indeed, as an example, a typical 18 KFT line can support approximately 300-500 kbps downstream. The region from about 30-138 kHz, if available to the downstream, would add about an additional 300-1000 kbps per wire pair for a total of about 600-2000 kbps. Yet an additional benefit of one embodiment of the virtual channel is the reduced need for echo cancellers in the ADSL transceivers.
Other advantages and features of the present invention will be apparent from the drawings and detailed description as set forth below.